In conventional GSM (Global System for Mobile Communication), a modulation scheme according to GMSK (Gaussian Minimum-Shift Keying) is implemented. GMSK is a constant envelope modulation scheme, where a phase shift is differentially dependent on the bit sequence. The GMSK modulation has been chosen as a compromise between fairly high spectrum efficiency and reasonable demodulation complexity.
EDGE (Enhanced Data for Global Evolution) is a high-speed mobile data standard, intended to enable second-generation GSM and TDMA (Time Division Multiple Access) networks to transmit data up to 384 kbps. EDGE provides the speed enhancement by changing the type of modulation used and making better use of the carrier currently used. It enables a greater data transmission speed to be achieved in good conditions, in particular near the base stations by implementing 8PSK (Eight-Phase-Shift Keying) modulation. The 8PSK modulation scheme is a high transmission modulation based on phase shift coding. The modulation is of a non-constant envelope type. EDGE can co-exist with the existing GSM traffic, switching to EDGE mode when appropriate.
When upgrading a base station to handle EDGE, the transmitter system has to be modified. A transmitter used for standard GSM purposes is designed for supporting GMSK, which means that the power amplifier that are used typically are more or less non-linear. When implementing 8PSK, the envelope may vary in a pre-defined way over time, and non-linear amplification can not be accepted. Thus, in a general case, a new parallel transmitter arrangement has to be provided. Since the transmitter devices are costly, parallel transmitter arrangements, which are only used one at a time, means a poor utilization of installed equipment. Furthermore, highly linear power amplifier elements or arrangements are very expensive and there is a request to avoid solutions using such elements.
In “Increasing the talk-time of mobile radios with efficient linear transmitter architectures” by S. Mann, M. Beach, P. Warr and J. McGeehan, Electronics & Communication Engineering Journal, April 2001, Vol. 13, No. 2, pp. 65-76, the relationship between linearizing methods for power amplification in radio transmitters and efficiency is discussed. LINC (LInear Nonlinear Component), known in prior art e.g. by U.S. Pat. No. 5,990,734, is one of the investigated schemes, where one non-constant envelope signal is divided into two constant envelope signals, which subsequently can be amplified by non-linear amplifiers. However, since such a method requires two non-linear amplifiers, this is not a particularly efficient approach for systems also handling constant envelope signals.